1. Field of the Invention
The present invention relates to an improved process for the preparation of solar salt from sea and sub-soil brines. More specifically the invention relates to a cost-effective process for preparation of common salt wherein both the absolute purity of the salt as well as the ratio of Ca2+ to Mg2+ are important, such as in chlor-alkali and soda ash industries. Still more specifically, the process relates to preparation of the above solar salt through an improved process wherein prior to charging of brine into crystallizers it is treated with alum to yield high purity salt as reported in the prior art followed by the inventive step of controlled adjustment of pH to also effect the desired ratio of Ca2+ to Mg2+ in the salt.
2. Background of the Invention
The world salt production has crossed two hundred million tons per annum. About 60% of the salt produced is used for industrial applications, chlor-alkali and soda ash industries being the major consumers. Superior quality industrial grade salt with specified Ca2+ to Mg2+ ratio is preferred by these industries as the use of such salt reduces the brine purification cost and effluent generation. 40% salt goes for human consumption where the caking of salt on storage poses great problems. This is attributed to the hygroscopic nature of the magnesium impurities present in salt. The lower magnesium content can also enhance the stability of iodizing agent in iodized salt.
Solar salt is produced using seawater, sub-soil brine and lake brine. All these brines constitute a multi-component salt system with a number of ions in the dissolved state and recovery of any of these salts in its pure form directly from these brines poses a problem. Hence salt produced from such brines is invariably contaminated with impurities such as Ca2+, Mg2+, SO42− and heavy metals. All these impurities are detrimental for industries where salt is used as a basic raw material. On the other hand, solar salt production from natural brines is most cost-effective. It is, therefore, important to devise means of making solar salt with minimum impurities while retaining the advantage of cost-effectiveness.
Reference may be made to the paper entitled “Primary Brine Treatment Operations” by D. Elliott presented at the 1999 Eltech Chlorine/Chlorate Seminar on Technology Bridge to the New Millenium, Ohio, 13 Sep. 1999, wherein the critical importance of salt purity and the deleterious effects of various contaminants including heavy metals on chlor-alkali manufacture are highlighted.
Brine caustic soda production results in brine mud, one of the largest waste streams from the chlor-alkali industry. About 30 kilograms of brine mud are generated for every 1000 kilograms of chlorine produced, but this varies with the purity of the salt used to produce the brine. The brine mud contains a variety of compounds, typically magnesium hydroxide and calcium carbonate, which are produced in the process of removing calcium and magnesium impurities in the salt through soda/lime treatment. Following the treatment the Ca+2 and Mg+2 impurities in the brine are reduced to ppm levels. The brine may then go through further purification utilizing ion selective resins.
Reference may once again be made to the article on Primary Brine Treatment Operations presented at the 1999 Eltech Chlorine/Chlorate Seminar on Technology Bridge to the New Millennium held at Cleveland, Ohio on Sep. 13, 1999, where in the importance of maintaining Ca2+ to Mg2+ ratio in salt is clearly explained. The higher magnesium content in salt poses the problem of settling the flocculent precipitates during the primary treatment of brine before its processing for chlor-alkali manufacture. It has been evidenced that the higher the magnesium content lower is the settling rates. Hence the chlor-alkali manufacturers insist on Ca2+ to Mg2+ ratio in range of 2-4. This above prior arts teach us that the absolute purity of salt as also the ratio of Ca2+ to Mg2+ are both important for a facile brine purification process with minimum waste generation.
Reference may be made to a large number of articles describing the manufacture of soda ash from salt by the Solvay process. Here again, brine purification through soda/lime treatment is of crucial importance.
Reference may be made to U.S. Pat. No. 7,037,481, dated May 2, 2006, wherein, Becenel, Jr. reports the methods and installations for producing ultra pure sodium chloride salt crystals primarily for use in saturating depleted brine resulting from the electrolytic decomposition of saturated brine in chlor alkali membrane cells for the production of chlorine, caustic soda and hydrogen. This invention particularly relates to the production of ultra pure sodium chloride salt crystals by processing primary treated brine by first acidifying the primary treated brine, then stripping the carbonic acid produced by acidification as carbon dioxide, and then returning the brine to a pH of about 6 or higher which is sufficient for processing it in evaporation equipment where the ultra pure salt crystals are produced.
In the article “Washing of Strip Mined Rock and Solar Salt at Leslie Salt Corporation US” (Symposium on Salt—I, Vol. 1, the Northern Ohio Geological society Incorporation, Cleveland (1961), p 449-464), A. Woodhill has reported that Ca2+, Mg2+, and SO42− impurities in solar salt can be reduced by mechanical washing. The main disadvantage of the method is that there is a 15-20% loss of salt and the method requires high capital investment. Moreover, the maximum level of reduction of Ca2+ is 70% and embedded impurities are difficult to remove.
In the patent application GB 20020028351 and 20021205 dated 9 Jun. 2004 entitled “Extracting Sodium Chloride From Sea Water Using Nano Filtration” by Kenny Conor et al. it is reported that sea water is pretreated to make it suitable for nanofiltration and the nanofiltered sea water is sent to a thermal desalination plant which operates as a sodium chloride concentrator and a distilled water producer. Sodium chloride is crystallized from the concentrated solution and the process provides a high purity sodium chloride suitable for many industries. It is claimed that the salt so produced eliminates many of the requirements of the primary and secondary brine treatment for the chlor-alkali industries. The nanofiltration process has a higher rejection rate for calcium, magnesium and sulphate ions as compared to Na+ or Cl− ions. The drawbacks of this process are that it would entail high capital investment and additional unit operations which would be uneconomical for standalone production of common salt in solar salt works. Moreover, it needs to be noted that whereas NaCl solubility in water is 35%, its solubility in brine is only 25% which means that advantage can be taken of the common ion effect to reduce NaCl solubility in brine which advantage would be lost if the divalent ions were to be completely removed by the process of nanofiltration and more time would be required for evaporation.
Reference may similarly be made to Central Salt & Marine Chemicals Research Institute's Biennial reports 2000-2002 and 2002-2004 wherein the purification of saturated brine through a nanofiltration process is reported.
In the patent application GB19540033194 19541116 dated Dec. 19, 1956 entitled “Improved Method of Preparing Sodium Chloride Brines of High Purity” Albright and Wilson have claimed that sodium chloride brines low in calcium sulphate content are prepared by dissolving solid sodium chloride contaminated with calcium sulphate in water in presence of a polyphosphate soluble in brine in the concentration range of 50-100 ppm. It is claimed that the amount of calcium sulphate is further decreased by dissolving solid sodium chloride in the presence of both the poly phosphate and water-soluble alkaline earth metal compound such as calcium chloride or acetate or barium chloride up to 1% level. The drawbacks of this process are that it is less appropriate for solar salt production and more appropriate as a means of post-treatment of brine obtained by dissolving salt.
Reference may be made to the European Patent No: EP 1,545,733,B9 (WO 2004/018068) dated Apr. 21, 1999 by Mayer et al. wherein an evaporative salt crystallization process that produces pure salt is disclosed. The process utilizes saccharide or its derivative in an evaporative process occurring at room temperature. The main disadvantage of the said process is that the saccharide is used in about 5% (w/v) concentration which would increase the viscosity of the brine and slow down evaporation and also add significantly to cost.
In their patent application (U.S. Pat. No. 3,891,297 dated Jun. 24, 1975) entitled “Crystallization of sodium chloride of reduced calcium sulfate content in presence of about 5 to about 500 ppm” by H. W. Fiedelman a process for the preparation of the cubic crystalline form of sodium chloride is described using either (1) a feed and bleed procedure comprising admixing an alkali metal phosphate with an aqueous solution of salt to increase the super saturation of calcium sulphate there in and evaporating the brine at an elevated temperature and reduced pressure to cause crystallization of pure salt and concomitantly bleeding brine from the chamber to the feed brine such as to maintain the calcium sulphate in the dissolved state and prevent its precipitation with salt or (2) subjecting the brine to solar evaporation to concentrate the same to the salt point, i.e., that point at which the salt will crystallize from the brine, adding an alkali metal polyphosphate to brine at this point to increase the super saturation of calcium sulphate there in and processing the brine for salt production following the conventional method. The process involves addition of costly chemicals at a very high dosage level. In the U.S. patent (U.S. Pat. No. 6,812,011 dated 2 Nov. 2004) entitled “An Improved Process for the Removal of Ca ions from the Brine by Marine Cyanobacteria” by S. Mishra et al. it has been claimed that common salt with reduced Ca2+ impurity can be produced from sea/subsoil brine by mopping up Ca2+ in the brine through certain types of marine cyanobacteria. The drawback of this process is that although the process has been demonstrated in small solar pans, it is not readily amenable to scale up. More over the process describes the method of reduction of Ca2+ impurities whereas the process does not describe the reduction of remaining impurities such as magnesium and sulphate.
In their paper entitled “Improvement in quality of salt from in-land brine of Kharaghoda area, India” (Research and Industry, Vol. 37, March 1992, pp 46-48), A. U. Hamidani and J. R. Sanghavi have explained a method of reducing the Ca2+ content in salt by establishing a common ion effect in the saturated brine by increasing the sulphate content through addition of either MgSO4 or Na2SO4. The drawbacks of the method are that though the Ca2+ content of salt is reduced, the Mg2+ and SO42− content cannot be reduced. Moreover, it involves compositional changes which are sometimes difficult from a logistics and cost point of view.
In the U.S. patent (U.S. Pat. No. 6,776,972 dated 17 Aug. 2004) entitled “A Process for Recovery of Common Salt and Marine Chemicals from Brine in Integrated Manner” by R. N. Vohra et al. it is claimed that common salt and marine chemicals of high purity can be recovered in an integrated manner by forced desulphatation of brine with inexpensive sources of CaCl2 such as distiller waste of Solvay Process prior to crystallization of salt. The process works well for any kind of brine and can also be carried out at large scale but the main drawback is the lack of availability of such calcium chloride source in the vicinity of most salt works. Another drawback of the process is that care must be taken to ensure that fresh brine does not mix inadvertently with desulphated brine in the crystallizer since the excess calcium chloride can form gypsum in the crystallizer that would deteriorate the quality of salt. Yet another draw back of the invention is that though the calcium impurities are reduced the magnesium content is not affected greatly by the distiller waste treatment. More over, a change in composition of brine is effected by the addition of distiller waste liquor.
H. M. Patel, in his research article that appeared in the Proceedings of 6th International Symposium on Salt, Vol. 2 pp. 515-533, has disclosed that Ca2+ and SO42− impurities in salt can be reduced using the difference in dissolution rate of NaCl and CaSO4. The main drawbacks of the process are that it employs unit operations like dissolver and chemical process reactor. It also requires addition of lime and soda for the removal of Mg2+ and Ca2+ and subsequent filtration of brine.
Reference may be made to the research article: “Rain Washing of Common Salt Heaps” by M. P. Bhatt et al. (Salt Research and Industry 10 (2), 1974, p 13) wherein it is reported that sea salt, as produced in solar pans contains 0.16-0.18% Ca2+, 0.3-0.4% Mg2+ and 0.70% SO42−, whereas after rain washing the salt contains 0.21% Ca2+, 0.06% Mg2+ and 0.60% SO42−. Although rain washing reduces Mg2+ impurities to some extent, the Ca2+ and SO42− impurities cannot be reduced from the harvested salt even by repeated washings. On the contrary, it is observed that the concentration of Ca2+ increases after rain-washing. The effect of magnesium impurities at reduced levels of calcium to maintain the Ca2+ to Mg2+ ratio in salt is not described in the article.
In the article “Manufacture of Solar Salt by Series Feeding System” by R. B. Bhatt et al. (Salt Research and Industry, 11, 1979, p 9) it has been reported that solar salt with less impurities of Ca2+ can be produced from sea water by a series feeding method wherein the salt is harvested in two stages i.e. between 25.5-27° Be′ (Sp. Gr. 1.214-1.230) and 27-29° Be′ (Sp. Gr.1.230-1.250). Salt harvested in the first stage is of a superior quality. Although this is a good process the drawback is that calcium and sulphate impurities cannot be reduced beyond a point even though higher levels of reduction are desirable. The article does not describe the processing of sub-soil brines which is deficient in sulphate content as compared to sea brine. More over the Mg2+ content in the second fraction collected between 27-29° Be′ (Sp. Gr.1.230-1.250) is found to be higher which can only be removed through mechanical washing involving losses of salt and additional cost.
In the Indian Patent No. 191912 (notified in the Indian Gazette) entitled “Preparation of Sodium Chloride Containing Low Ca Impurity from Sea Brine in Solar Salt Works” by J. R. Sanghavi et al. it is claimed that addition of a polysaccharide additive namely starch in concentration of 50-150 ppm into concentrated brine can reduce calcium impurity in salt to less than 0.05-0.1 percent as Ca2+. The drawbacks of the process are that it requires addition of hot solution of starch which is both cumbersome and costly, addition has to be repeated several times and no mention is made of the effect of the treatment on other impurities in salt. No explanation is also provided for the origin of the observed effect. Moreover the magnesium content of salt cannot be reduced by the cited process.
In their patent application (WO 2004069371 dated 19 Aug., 2004), Kamishima Hiroshi et al. have claimed that sodium chloride crystals with reduced impurities can be produced from aqueous sodium chloride solutions by passing the solution through a column packed with an adsorbent on to which the impurity is selectively adsorbed. The method also provides a sodium chloride composition for preparing artificial seawater for use in algae cultivation, which is reduced in Mg2+ ion or Ca2+ ion concentration. The drawbacks of the process are that it is not applicable to a multi-component system like sea/sub-soil brine. This process does not give any clue about the production of superior quality salt directly from sea/sub-soil brine in a solar salt works.
In the patent (U.S. Pat. No. 4,072,472 dated Feb. 7, 1978) on High purity salt from high sulphate salt deposits by A. Lukes Jerome it is reported that subterranean salt deposit is solution mined, and the resulting calcium- and sulphate-contaminated brine is treated with soda ash to precipitate calcium compounds. After settling the slurry the clear brine is evaporated in a series of solar ponds to produce high-grade sodium chloride. This process is not economically feasible for large solar salt works where salt is produced from sea/sub-soil brines. Moreover, the process removes only calcium content from salt and the magnesium and sulphate impurities remain unaffected.
In the U.S. Pat. No. 3,647,396 dated 7 Mar. 1972 entitled “Production of High Purity Salt”, H. W. Dewittie et al. have claimed to have developed a process for the recrystallization of sodium chloride in the form of high purity cubic crystals from a sodium chloride source containing calcium sulphate impurity by multi-effect evaporation preceded by treatment of the hot sodium chloride saturated brine by flocculants and settling, to cause the undissolved calcium sulphate particles and other suspended solids to agglomerate and settle out of the brine prior to recrystallization of sodium chloride eliminating the conventional requirement for filtering the hot brine. The main drawbacks of the process are that it involves recrystallization which is expensive, time consuming and energy intensive. There is no mention of the utility of the method for production of pure salt directly from sea brine or sub-soil brine in solar salt works.
AJK Environmental Specialties, Inc., Marchant Ville, N.J., USA has also come out with a commercial flocculating agent by the name of Aquasorb which is a cross linked polymer comprising solely of sodium polyacrylates. It is claimed that the product works well for brine clarification and removal of Ca2+ and Mg2+. Qumi International, Inc., Texas, USA has also claimed to have produced such polyacrylamide-based flocculants and coagulants for similar purposes. These flocculating agents are used to reduce residual Ca2+ and Mg2+ impurities in brine, such brine being thereafter used directly in industrial applications such as in chlor-alkali and soda ash industries. The reported process is specific to the treatment of brine prepared from the harvested salt and may not work directly with concentrated sea brine or sub-soil brine for the production of solar salt with highly reduced levels of calcium and magnesium impurities.
In patent No. WO2007036949 dated: Apr. 5, 2007, entitled “a cost-effective process for the preparation of solar salt having high purity and whiteness” Mukhopadhyay Indrajit et al. have claimed to have developed a cost effective process for the production of high purity solar salt (>99.5% purity on dry basis after heap washing) with improved whiteness having highly reduced levels of calcium and sulphate impurities and also of heavy metal ions. Although the process as developed has many advantages in terms of ease of operation and eliminating the calcium and sulphate impurities to the minimum possible levels, magnesium impurities, unfortunately, are not reduced in the same proportion as a result of which the Ca2+ to Mg2+ ratio in the salt is <1 and, consequently, fails to meet the preferred ratio of 2-4 desired for soda/lime treatment for brine purification in chlor-alkali and soda ash industries as alluded to above.
It is evident from the prior art that there are drawbacks in all of the prior arts, especially as applicable to solar salt production in the field. There are many processes where a salt of high purity is obtained but the processes are either cumbersome or costly and not practical for implementation in the field. There are other processes where the desired Ca2+ to Mg2+ ratio is obtained easily in the field but the absolute purity of the salt is poor. On the other hand, treatment of brine with alum to produce high purity salt as reported in the recent prior art is an attractive process for production of solar salt in the field, with purity >99.5% on dry basis. But, here again, the process suffers from the important drawback of yielding salt having undesired ratio of Ca2+ to Mg2+ because of which chlor-alkali and soda ash industries may be reluctant to use the product. It is therefore highly desirable to improve the alum treatment process to obtain salt with desired Ca2+ to Mg2+ ratio while retaining all of the obvious advantages of the process.